docs: def eval_expression(value):
      """ Evaluate a full time expression """
      start = eval_function(value.ts_expression[0])
      interval = eval_interval(value.ts_expression[2])
      op = value.ts_expression[1]
      if op == "+":
          return start + interval
      elif op == "-":
          return start - interval
      else:
          raise SyntaxError("Unrecognized operator %r" % op)